Check length of a String - c++

Question

I'm a beginner to C++. We are doing a project where we input firstName, lastName, and SSN for employee. Here what I have done:
#include <iostream>
#include <stdexcept>
#include "Employee.h"
using namespace std;
Employee::Employee(const string &first, const string &last, const string &ssn)
{
firstName = first;
lastName = last;
SSN = ssn;
}
void Employee::setFirstName(const string &first)
{
firstName = first;
}
string Employee::getFirstName() const
{
return firstName;
}
void Employee::setLastName(const string &last)
{
lastName = last;
}
string Employee::getLastName() const
{
return lastName;
}
void Employee::setSSN(const string &ssn)
{
if (ssn.length() == 9)
{
SSN = ssn;
}
else
{
cout << "Please enter SSN again: " << endl;
}
}
string Employee::getSSN() const
{
return SSN;
}
void Employee::print() const
{
cout << "Employee: " << getFirstName() << ' ' << getLastName()
<< "\nSocial Security Number: " << getSSN();
}
My instructor wants us to check the length of the SSN (simplest way) to make sure it 9 digits, and if it's more or less, ask users to input again. I do not know how to validate the input for SSN. Can anyone help me please?

Answer 1

std::string::length() is the function to be used to find length and std::string::empty() to check weather string is empty or not.
if(SSN.length() < 9 && !SSN.empty())
{
//Need C++11 and above for this kind for loop
for(auto &x: SSN)
{
if(std::isdigit(x))
{
//valid SSN
}
}
}

Answer 2

A compact solution to this problem:
bool ValidSSN(const std::string& ssn) {
if (ssn.size() != 9) return false;
return ssn.find_first_not_of("0123456789") == ssn.npos;
}

Answer 3

There is no simple and easy solution to checking the string for 9 digits.
My recommendation is a two step process:
Verify length is 9.
Verify string is an integral number.
Note: The code in the else clause does not work for cases such as "123Apple4". Thanks to #Steephen for the enlightenment. Leaving the code in the answer to synchronize with comments.
In code this would be:
bool is_valid = true;
if (ssn.length() != 9)
{
is_valid = false;
}
else
{
std::istringstream ssn_stream;
ssn_stream.str(ssn);
unsigned int value;
if (!(ssn_stream >> value))
{
is_valid = false;
}
}
Another alternative is to use a loop:
if (ssn.length() != 9)
{
is_valid = false;
}
else
{
is_valid = true;
for (unsigned int i = 0U; i < 9; ++i)
{
if (!isdigit(ssn[i]))
{
is_valid = false;
break;
}
}
}
Edit 1: Regular Expressions
If your compiler supports C++ regular expression, you could come up with a regular expression to define the SSN. You would then check if the contents of the string matched the regular expression.